The present invention relates to a method for recovering unreacted monomers, for instance, after suspension polymerization of a vinyl chloride monomer.
Regarding the preparation of vinyl chloride polymers, it has been desired to speed up polymerization cycles for the purpose of enhancing the productivity thereof and as a part of the speeding up of the cycles, it has been tried to increase the rate of recovery of unreacted monomers and to thus speed up the rate of recovery.
However, a simple improvement in the rate of recovery of the unreacted monomers is accompanied by foaming of a polymer slurry and in the worst case, foams containing the polymer even reach the gas phase zone in a polymerizer and pipings which are connected thereto. This leads to formation of scale and generation of fish eyes during the subsequent preparation of polymers due to contamination with the polymer particles remaining in the polymerizer and this in turn results in the reduction of quality of the polymers subsequently prepared. Moreover, in the worst case, the polymer slurry is carried over even to lines for recovering unreacted monomers. As a result, the yield of the polymer is substantially lowered and the recovery lines are sometimes clogged. For this reason, the rate of recovering unreacted monomers is selected in such a manner that foams of a polymer slurry do not fill the gas phase zone in the polymerizer and such a choice requires high skill.
However, the foaming condition of a polymer slurry while recovering unreacted monomers is greatly influenced by the pressure in a polymerizer and the composition of a polymerization system and, therefore, it has been very difficult to determine the optimum rate of recovery for all of the products and kinds thereof. Under these circumstances, there has been a demand for the development of a method for recovering unreacted monomers while inhibiting carrying over of a polymer slurry through foaming during recovering the unreacted monomers and shortening the time required for the evacuation of a polymerizer.